Utilising nuclear magnetic resonance (NMR) has become a standard procedure for noninvasively acquiring image data from inside a patient's body, e.g. the brain, by means of clinical MRI equipment. In this context, a technique using magnetic field gradients is employed which permits to obtain NMR signals (constituting NMR data) from specific locations in the patient. Typically, the region which is to be imaged (region of interest, ROI) is scanned in planes by a sequence of measurement cycles. The resulting NMR data are digitized and further processed to reconstruct the image using one of a variety of reconstruction techniques known to a person skilled in the art.
Prior art methods and systems have introduced the concept of geometry sharing with so-called “named geometries”. In this context, a user of the MRI equipment defines a number of scan plane positions, hereinafter referred to as scan geometries, having a unique identifier, e.g. a name, such as “TRA”. Upon acknowledgement of the scan geometry by the user, which is required only once, the system executes all scans with a common geometry, i.e. a common identifier, which accordingly share the same orientation relative to the patient, but usually employ different MR contrast settings. More specifically, in accordance with the known methods as disclosed, e.g., in prior art document NL 031229 C, so-called scout images are acquired first, based on which the user defines the scan geometries. Using said orientations, the MRI system then defines the corresponding scan parameters, e.g. magnetic gradients, and submits a scan to be performed to a dedicated control unit. After acquisition of a particular image frame, the latter is reconstructed and usually written to a database. Then the next scan is submitted, until all scans are finished. In this way, prior art document NL 031229 C discloses the use of “geometry sharing”.
However, this approach suffers from the following disadvantage: During routine scanning patients do often move involuntarily. However, the known method and system assumes that patients not move at all, or only in a minimal way, since the scan geometry and the corresponding scan parameters are only defined once, and never updated afterwards. This may cause a geometrical mismatch between the acquired ROI, i.e. a scan slice relative to anatomy, and the intended ROI with respect to subsequent scans, i.e. queued scans ready for dispatching from the scan queue to the scanner, which should ideally share the same geometry. When reading multi-contrast scans, this implies that a radiologic reader has to reorder scans to match geometries, which is a costly and time-consuming process.
Prior art documents WO 01/84172 A1 and CA 2 473 963 A1 tackle the problem of motion correction in connection with MRI techniques such as functional MRI (fMRI), wherein each individual scan is performed as a time series of individual scans which are equal in terms of MR parameters and contrasts, i.e. each scan includes a plurality of separate image frames, and the patient moves between and/or during capture of such image frames. This is also referred to as “time-sliced” image acquisition. To this end, a current patient motion is repeatedly measured during the scan, and at least one scan parameter of an MRI system is adjusted accordingly prior to performing the next scan of the time series. More specifically, as stated before a user defines scan geometries from scout images, which in turn are used to define the scan parameters. Then a (time-sliced) scan is submitted and a given image frame N belonging to that scan is acquired. After reconstruction of said image frame N, the latter is registered versus an image frame N−1 of the same scan that has been acquired earlier in the time series. The registration result is used to derive a transformation matrix T which is used to update the scan parameters for a subsequent image frame N+1. Thus, updating is an exclusively local process, which implies that image registration is performed on frames with similar contrast and involves modification of the RF excitation frequencies and gradient waveforms only. In the context of shared geometries, in this approach there is no relation between subsequent scans (i.e., respective time series) with the same geometry. Again, this may cause a geometrical mismatch between an acquired and an intended ROI for subsequent scans, which should ideally share the same geometry.
Thus, there is a need in the art for a method and a system which ensure that subsequent scans with a common predefined geometry do effectively lead to the acquisition of image data which share a physical geometry relative to a patient.
It is the object of the present invention to provide a method for acquiring image data from a patient with a magnetic resonance imaging (MRI) system which obviates the above-mentioned disadvantages. It is also an object of the present invention to provide magnetic resonance imaging (MRI) system which obviates the disadvantage that subsequent scans with a common “named geometry” effectively acquire data from differing regions in the patient, which usually do not coincide with an intended ROI due to patient motion. Furthermore, the present invention has for its object to provide a computer programme product adapted to translate into action the above-mentioned method in accordance with the present invention.